Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

c) Will A reduce B to B

2-;

i.e.

, is A + B

→

A

2+ + B

2- spontaneous?

Example 11.1c

Example 11.1d

A

B

2+A

2-B

Reactants

Products

Reactants

Products

A

2B

2+

4+A

2B

The empty orbital on B is lower in energy

than the high-energy electrons of A (green

circle), so the electrons transfer spontaneously

from A to B. B is a strong enough oxidant

to oxidize A to A

2+. Alternatively, A is a strong enough reductant to reduce B to B

2-.

d) Will B

2+ oxidize A to A

4+;

i.e

., is A + 2B

2+^

→

A

4+ + 2B spontaneous?

Both valence orbitals are empty in the B

2+ ion, and they are lower in energy than all of the

valence electrons on A, so all four

electrons transfer spontaneously. One B

2+ would

accept the high-energy pair (A + B

2+^

→

A

2+ + B), and a second B

2+ would accept the low-

energy pair (A

2+ + B

2+^

→

A

2+ + B). Thus, B

2+ is a strong enough oxidizing agent to oxidize

A to A

4+.

The number of electrons gained during the

reduction, the number of electrons lost

during the oxidation, and the number of electr

ons transferred during the redox reaction are

all the same number, n. For example, the reaction between Cu

2+ and Fe involves a gain of

two electrons by Cu

2+, a loss of two electrons by Fe, and is a two-electron transfer from Fe

to Cu

2+;

i.e.

, n = 2 electrons. To determine the number of electrons transferred,

1. Identify the oxidant and reductant. 2. Determine the oxidation st

ate changes of the atoms invo

lved in the oxidation and

reduction.

3. Multiply the oxidation state change of the atoms involved in

the oxidation or reduction by

the number of atoms involved in the proc

ess to obtain n, the number of electrons

transferred.

Redox reactions often involve ions that are not involved in the electron transfer, and

the species that are involved can be polyatomic ions or molecules with several elements, so the identities of the oxidant and reductant

are not always obvious. Although, the change

in oxidation number is attributed to a single atom in the molecule or ion, the entire ion or molecule is considered to be the oxidizing

or reducing agent. The following guidelines can

be helpful in identifying the atoms whose oxidation states* are changing:

* Oxidation states were discussed

in Section 4.4.A review of that

material would help your understanding here.

a. Atoms appearing in elements are

always being oxidized or reduced.

b. O and H often appear as H

O, OH 2

1-, and/or H

1+, but they are not involved in the electron

transfer unless they are elemental (O

or H 2

), hydride (H 2

1-), or peroxide (O

2- 2

).

c. Most transition elements can exist in severa

l oxidation states, so they are involved in

many redox reactions.

d. The central atoms of polyatomic ions fr

equently undergo oxidation or reduction.